When an oil or gas well is completed it may be necessary to install tubulars designed to control solids that may be produced as a result of the production of hydrocarbons or water. Common types of tubulars that may be used are pipes with slots or holes alone or in combination with a metal mesh or wrapped wire with defined apertures. Additional methods of solids control include prepacked sand, porous matrix around the tubing or placement of specific designed particles as a filter medium i.e. sand or gravel; commonly referred to as gravel packs. However, in all cases a lower completion which has apertures in its side wall needs to be installed in the open hole region of the well.
The lower completion is normally run on drill pipe with an inner string inside the lower completion, called a wash pipe, that allows circulation through the drill pipe and wash pipe exiting at the base of the completion. A fluid path through the tubing may be required, for example, to drive a motor at the end of the tubing, allow circulation to assist with moving the tubing to the bottom of the hole, drive other devices to enable movement of the tubing by reducing friction i.e. agitators, create circulation to remove any undesirable fluids that may enter the wellbore, place other fluids in the wellbore that may be required as part of the operation, place the chemical treatment to remove the material on the face of the wellbore or place the filter medium.
The primary purpose of the wash pipe is as a conduit for completion fluids to enable the lower completion to be washed to the bottom of the hole and also for placement of a filter cake breaker or other fluids.
The filter cake breaker is a chemical designed to remove the filter cake material placed across the formation during the drilling process as this may inhibit the future flowing productivity of the reservoir formation, as well as block the apertures of the mesh, slots or holes in the lower completion when the oil and/or gas is produced. If the filter cake is not removed, then it could dislodge from the sand face (the formation) during production and plug the production apertures in the lower completion causing production impairment. The filter cake breaker may also be required to remove any damage to the production matrix that occurred during the drilling process.
The chemicals used for material removal could be; acids, acid forming chemicals, solvents, surfactants, enzymes, oxidizing agents, catalysts, polymers, brines, biocides, corrosion inhibitors.
Once the lower completion, including any solids control tubing, is placed in the required position within the wellbore, and any chemical treatment conducted, it is then necessary to recover the drill pipe/inner stubbing (wash pipe) installation string to the surface, which includes removal of the inner tubing (wash pipe) to enable fluid to pass from the wellbore into the lower completion, via the solids control tubing if present. The chemical treatment needs to be aggressive enough to adequately remove the filter cake material placed across the formation during the drilling process, however this often comes with an undesirable consequence of reacting very quickly, resulting in fluid from above the completion being on continual losses to the area where the material has been removed.
It is common that the fluid that is above the lower completion is brine or water however it can also be the original fluid used to drill the wellbore or other fluids required as part of the operation. The brine or water contains additional chemicals that may include but not limited to, lubricants, biocides, oxygen scavengers, corrosion inhibitors.
Whilst the rate of loss of this fluid can remain within acceptable limits, this is not always the case. In fact, losses have always been a major problem and can be both costly and have well control implications.
Fluid being lost to the zone may cause damage to the zone thereby impairing movement of the hydrocarbons into the wellbore. If losses occur then there are limited options that can either lead to a) further formation damage, b) additional workover costs, c) loss of the well. Remedial steps to control loss may include placement of material back into the wellbore that will plug the formation to which the losses are going, or plug the holes, slots or mesh of the tubing.
It is preferable not to have to take remedial steps and therefore a method of preventing the losses when removing the wash pipe is needed.
During the completion design process it is possible to add ‘safe zones’ that enable the wash pipe to be pulled to a point where the annulus can be isolated thereby stopping the loss. A ‘safe zone’ is a zone that has a smooth metal face in the lower completion where rubber seals mounted on the wash pipe can engage to form a seal from the annulus.
These safe zones are only used to enable remedial material to be prepared or to replenish fluid supplies. Most drilling contractors will not allow the wash pipe to be pulled if the well is on losses.
A wash pipe is commonly 5,000 feet long and may take some 5 hours to withdraw. For this reason relatively mild breaker chemicals are used, which will take at least five hours following the end of circulation of the fluid to completely remove the filter cake. In this way, substantial fluid losses are prevented since the filter cake is not removed until after the wash pipe is withdrawn. However, more aggressive breakers are preferred which break down the filter cake in as little as half an hour or less after circulation has finished. Aggressive breakers remove the filter cake more reliably which can result in improved production. Wash pipes can be as short as 2,000 ft or can be much longer than 5,000 ft.
There is therefore an unmet need for some way of using aggressive breaker chemicals without risking excessive fluid losses.
There have been many designs proposed which provide a way to seal the annulus between the wash pipe and the lower completion at a given point or over a short length of the wash pipe (“safe zones”). See, for example, Clarkson, et al., “Evolution of Single-Trip Multiple-Zone Completion Technology: How State-of-the-Art New Developments Can Meet Today” SPE Annual Technical Conference and Exhibition, 21-24 September, Denver, Colo., USA (2008). Other approaches to controlling fluid loss have included placing a sealing material over the sand screen apertures which may be removed by chemical means—see U.S. Pat. No. 7,204,316 (Dusterhoft).